An improved encased high temperature optical fiber

ABSTRACT

An apparatus and method for measuring the tip location of a consumable electrode in an electric arc furnace employing an optical fiber with high temperature capability and an optical time domain reflectometer. The optical fiber is encased with a metal tube to prevent the optical fiber from slipping therein and to provide high temperature capability.

This is a division of application Ser. No. 07/177,751 filed Apr. 5,1988, now U.S. Pat. No. 4,843,234.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to measuring the consumableelectrode length in an electric arc melting furnace. More particularly,it relates to measurement of the tip location of a consumable electrodeusing fiber optic technology and providing high temperature capabilityto an optical fiber.

2. Description of the Related Art

In the steel industry, measurement of the length of the consumableelectrode in electric arc furnaces is necessary to ensure minimumenergy, material losses, and prevent shortout in the hot metal. In asubmersible arc application, the tip of a consumable electrode isimmersed in a pile of ore at some predetermined point. There is an arcproduced which forms molten metal that flows out of the furnace througha slanted bottom or some regulated draining system that is well known inthe art. It is important to know the exact tip location of theconsumable electrode for optimal production.

To obtain the best results when using an arc operating device such as anarc furnace, or an arc welder it is imperative that the distance betweenthe elements of the machine between which the arc forms is adjusted andcontrolled.

In the application at hand, it is compulsory to maintain the length ofthe arc at its optimal value by controlling the distance between theelectrode and the molten metal especially when the electrode iscontinuously moving due to erosion of its tip.

Some methods require a manual "dipstick" or "sounding" measurement whichare inefficient and non-continuous. However, process modeling requires acontinuous input of the location of the electrode tip for processoptimization. This position information is used not only to readjust tiplocation but to control, for example, the feed rate of fresh mineralsand drain rate of molten metal. Inputs which are provided to the processmodel based on the infrequent manual soundings, and a prior knowledge ofelectrode consumption rates, are known to have inaccuracies andcumulative errors.

There have been many attempts in the prior art to find a way toaccurately and precisely control and measure the position of theconsumable electrode as it is being consumed in the furnace. Most ofthese attempts focus on the use of arc voltage or some electricalphenomenon generated from the arc voltage such as "hash" and "dropshort" phenomena. One such reference, U.S. Pat. No. 4,303,797 employs amathematical model based on electrode drive speed and voltagediscontinuities to control the gap between the bottom of an electrodeand the top surface of an ingot. A similar approach was used byKjolseth, et al (U.S. Pat. No. 3,375,318) by employing the derivative ofchange in resistance with respect to the electrode position asdetermined for the location of the electrode point. Both of thesereferences teach the use of some electric variable for setting up amathematical model for controlling the position of a consumableelectrode. Likewise, U.S. Pat. No. 3,187,078 teaches of using voltagediscontinuities in a servo system for controlling the arc gap and U.S.Pat. No. 4,578,795 discloses a process for monitoring the gap voltagebetween the consumable electrode and the molten metal by monitoring theoccurrence of drop shorts. The problem with these approaches is thedifficulty of precise control due to the electrical noise. The problemsencountered using an electrical approach are discussed in the previouslycited references.

Even earlier approaches to monitoring the length of a consumableelectrode included a signaling device consisting of a wire extendingfrom a reel to the electrode. When the electrode is consumed up to thewire, the connection is broken causing the lamp, to which the wire isalso connected, to go out. This signals the operator that the meltinghas progressed to the predetermined point. A slightly different approachin U.S. Pat. No. 3,379,818 used the weight of the remaining electrode asa signaling device.

A more recent attempt for controlling the length of an electrical arc inan arc generating machine employed acoustical signals in U.S. Pat. No.4,435,631. This reference teaches that the acoustical signal generatedby an arc is a function of the length of its column which can becompared to a reference signal. It also teaches to modulate the arccolumn supplying current, for a DC current, to generate an acousticalsignal. This reference points out several drawbacks with the prior artfor measuring arc voltage drop via an electrical voltage probe. Thefirst drawback is that the electrical voltage probe is neverelectrically insulated from the arc power supply. This poses noise anddrift problems when the arc generating machine is operating at highvoltage or when the arc supplying current is floating. A second drawbackis that the measuring loop is subject to parasitic voltages induced inthe loop when the arc current undergoes large variation.

This reference also teaches of the difficulty of obtaining an exactvalue of the arc voltage drop. It teaches that it is almost impossibleto isolate the arc column voltage drop from the value obtained with themeasuring loop.

The disadvantages of employing an electrical approach for control extendto an acoustic approach that uses magnetostrictive wire. Sinceelectrically conductive wire must extend down to the electric arc, thereis still the presence of electrical noise. Likewise, the acoustictime-of-flight depends on wire temperature, which is unlike the presentinvention. Finally, distance, resolution and precision depend on pulsewidth. Optical pulses can be produced with picosecond width, whileacoustic pulses cannot. To use acoustical signals in both AC and DCapplications, additional equipment is required.

There is a need for an apparatus and method for measuring the length ofa consumable electrode which has electrical noise immunity. There isalso a need for an apparatus that provides an electrically noise-freelink to the electrically hostile arc furnace.

Optical fiber length measurement using an optical time domainreflectometer (OTDR) is a method used primarily for fault or breaklocation such as taught in U.S. Pat. No. 4,289,398, which is herebyincorporated by reference. It has not been suggested before to applythis technology to a consumable electrode tip measurement.

SUMMARY OF THE INVENTION

The present invention provides a new method and apparatus formeasurement of a consumable electrode's tip location to verify that thetip is properly positioned in an electric arc furnace. At least oneoptical fiber of a known length is introduced into the consumableelectrode. As the consumable electrode is melted in the furnace, theoptical fiber is melted therewith. The change in the length of theoptical fiber is measured and from those measurements the length of theconsumable electrode is calculated which is then used to determine thetip location of the electrode. Optical fiber technology permitsmeasuring the flight of a light pulse which originates at the fiber'sinput end and then is reflected back from the melted fiber end whichcorresponds to the end of the consumable electrode.

Advantageously, a second optical fiber, or even a plurality of fibers,with a known length may also be introduced into the consumable electrodefor the purpose of instantly verifying the length of the electrode. Bycutting one of the optical fibers, the instantaneous change in length ofthe cut fiber is equal to the distance between the fiber tip (at theelectrode tip) and the point at which it is cut.

Accordingly, one aspect of the present invention is to provide a meansfor controlling and measuring the length of a consumable element whichis immune to electrical noise.

Another aspect of the present invention is to provide a method foraccurately and precisely determining the tip position of a consumableelectrode in electric arc furnaces.

Advantageously, the foregoing aspects are achieved with an optical fiberhaving high temperature capability which lends certainty that theoptical fiber reaches the tip of the consumable electrode.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming partof this disclosure. For a better understanding of the present invention,and the operating advantages attained by its use, reference is made tothe accompanying drawings and descriptive matter in which a preferredembodiment of the invention is illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic cross-sectional view of a consumable electrodefurnace including a block diagram of the components of the preferredembodiment of the present invention.

FIG. 2 is a perspective view of an encased optical fiber with portionsremoved taken from II--II in FIG. 1.

FIG. 3 is a graph illustrating measurements of length using the presentinvention versus hand measurements of length.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The operation of a consumable electrode furnace is a dynamic process.FIG. 1 illustrates schematically the major components of a consumableelectrode furnace with some of the advantages of the present invention.The electrode 10 is being consumed as it is being lowered to compensatefor its erosion or consumption. The molten metal 12 is being formedwithin some type of crucible or furnace 14. The rate at which thesurface of the molten metal 12 rises affects the speed at which theelectrode 10 is lowered. As is known in the art, some means for drainingthe molten metal is necessary so as to keep the electrode tip optimallylocated at some fixed relative external point, P. This conventionaldraining system is designated by the slanted bottom 36 in furnace 14with a drain 38. The electrode drive means 16 is operated by some motorcontrol which is conventional to the art of electric arc furnaces. Thefurnace 14 and the rate for controlling the level of the molten metal 12are all known in the art. The focus of the prior art has been onadjusting the electrode drive means 16 so that it adjusts the electrode10 feed rate to maintain some constant arc length as is depicted by the(A) with the arrows showing the arc length in FIG. 1. Arc, (A), isproduced from power source V which connects electrode 10 to the moltenmetal 12. This arc length (A) can be determined from the length of theconsumable electrode relative to some external reference point, P, inthe melting furnace.

Alternatively, in a submersible arc application, ore is piled around thetip of the electrode while the molten metal is continually drained fromthe bottom. As the tip of the electrode 10 is being consumed, it isimperative that the tip's location is known so that the electrode drivespeed is at the proper rate. This tip position information can be usednot only to readjust tip location of the electrode 10 but to control,for example, the feed rate of fresh minerals or ore and drain rate ofmolten metal 12. Currently, input is provided to the process model basedon the infrequent manual soundings and the prior knowledge of electrodeconsumption rates. These methods are known to have inaccuracies, andcumulative errors arise from assumed consumption rates.

Since the electrodes are continually consumed in the electric arcfurnace, these electrodes must be continually regenerated. Preferably,the electrode is graphite. Ordinarily, regeneration is done outside thefurnace. For example, a graphite consumable electrode is continuallymolded as it is being consumed in the furnace. A carbon filled epoxy isplaced into a mold which is designated as 18 in FIG. 1 at the top of theelectrode 10 above the furnace 14. The epoxy hardens as the electrode 10is continuously lowered into the furnace 14 by the electrode drive means16 which maintains a proper tip depth. A spool of optical fiber of aknown length 20 is located above the electrode 10 and is connected to anoptical time domain reflectometer (OTDR) 22. The encased optical fiber24 from the spool 20 is introduced into the carbon epoxy at the top ofthe mold 18 for electrode regeneration. When the tip of the fiber 24reaches the tip of the electrode 26, the fiber melts away as does theelectrode tip 26. By measuring the rate of change of fiber length theburn rate of the electrode is determined.

If a second spool of optical fiber 28 is located above the electrode 10as shown in FIG. 1, the electrode tip position 26 may be instantaneouslyverified periodically by cutting one of the fibers 24 from either spool20 or 28. The instantaneous change in length of the cut fiber is equalto the distance between the fiber tip (at the electrode tip 26) and thepoint at which it was cut designated as C in FIG. 1.

For example, the following formulas are used in the calculations:

    L(t)=L(t.sub.F)+[F.sub.i (t.sub.F)-F.sub.i (t)]-S[t.sub.F =t](1)

    L(t.sub.F)=Fj(t.sub.b)-F.sub.j (t.sub.F)-1                 (2)

where:

L(t)=desired electrode length as a function of time

Fn(t)=measured length of optical fiber 24 from spool 20 (n=1) or spool28 (n=2)

S=known electrode drive speed (length per unit time).

Fn(t_(b))=measured length of optical fiber 24 from spool 20 or 28 justbefore cutting at time t_(6b)

Fn(t)=measured length of optical fiber 24 from spool 20 or 28 just aftercutting at time t_(F)

L(t_(F))=measured electrode length just after cutting of optical fiber24

1=known distance between point C and top of electrode, as shown in FIG.1.

It is known in the art that the OTDR 22 converts time of flight of alight pulse to distance which is used for the above-mentionedcalculations. The data from the OTDR 22 is input into the microprocessorcontrol 20 through transmission line 32. The microprocessor 30 employsthis data to direct through line 34 the electrode drive means 16 motorcontrol to maintain the proper tip location of the electrode 10 relativeto the external reference point P.

Due to the high temperature present at the tip of the electrode 10,there are problems with using optical fibers which have a plastic buffercoating. There is uncertainty whether the fiber actually reaches the tipof the electrode 26, because the plastic coating readily melts. A basicassumption of the present invention is that the encased optical fiber 24and the electrode 10 are consumed at the same rate.

There exists a metal buffer coating for optical fibers which iscommercially available from Hughes Aircraft, for example. The coating isan aluminum buffer coating. It is preferably to use this type of opticalfiber since it does not readily melt like the plastic coating.

The metal buffer coated optical fiber is not available as a cable whichcan be wound on a spool. Therefore, it has been found that to improvethe mechanical strength of a metal buffer coated optical fiber, athin-walled tube is swaged about the fiber. FIG. 2 depicts an opticalfiber 24 with a silica core 42, a doped silica cladding 44, and a metalbuffer coating 46. The optical fiber is inserted and threaded through athin-walled tube 48 such as about a 0.030 inch stainless steel tubingwith an internal diameter of about 0.015 inch to form the encasedoptical fiber 24. An optical fiber has an outer diameter of about 0.010inch so it fits easily and securely within tube 48. It is readilyapparent that the tubing can be composed of any metal as long as it isreasonably high melting with sufficient ductility to wrap on a spool.The tubing 48 is swaged onto the optical fiber. This term of artgenerally means that the tubing 48 is slightly crimped on the opticalfiber so as to retain it securely without crushing it. The soft aluminumcoating yields during swaging so that the fiber is prevented fromslipping in the tube, yet minimal stress is introduced into the fiber.Alternately, the space between the fiber and tube could be packed withalumina (Al₂ O₃) powder prior to swaging to minimize thermal expansioninduced stresses in the cable assembly.

Advantageously the encased optical fiber 24 now has high temperaturecapability which ensures the fiber 24 reaching the tip of the electrode26.

FIG. 3 is a plot of field test data taken on an operating graphiteelectrode furnace. The graphite electrode contained a hollow pipe toenable sounding meaurements to be made down to the molten metal. Thehorizontal scale or the X-axis represents hand measurements of length asan optical fiber was lowered into the pipe down close to the graphiteelectrode tip. The vertical scale represents date measured using theOTDR to obtain fiber length as the end of the fiber was vaporized. Ifthe two measurements were always in agreement, the data points would alllie on a 45° line. The relative measurement precision and cumulativeerror are about 6 inches. This data indicates the feasibility of makingreal-time measurements of graphite electrode length and tip position inelectric arc furnaces. In addition, sounding measurements were madebefore and after the fiber measurements. The sounding method requiredmeasurement of the length of steel cable as an asbestos bob was loweredthrough the pipe in the graphite electrode. With this method, therepeatability was 43 inches. The advantages of the fiber lengthmeasurement are thus real-time capability and improved precision.Another significant advantage is the optical fiber electrical noiseimmunity. The glass optical fiber is non conducting and thus provides anelectrically noise-free link between the electronic OTDR and theelectrically hostile electric arc furnace.

While a specific embodiment of the present invention has been shown anddescribed in detail to illustrate the application and principles of theinvention, it will be understood that it is not intended that thepresent invention be limited thereto and that the invention may beembodied otherwise without departing from such principles. For example,while the preferred embodiment has shown only two optical fiberssituated in the graphite electrode, it is possible for a plurality ofoptical fibers to be situated in an electrode for facilitating theverification of the length of the electrode at any point in time.Likewise, even though the preferred embodiment shows a graphiteelectrode being molded in place, based on the above it is readilyapparent that other compositions of the electrode may be moldedimmediately above the top of the electrode as it is continually beingconsumed with optical fibers situated therein.

What is claimed is:
 1. An encased optical fiber, comprising:an opticalfiber having a core, a doped cladding, and a buffer coating; and a metaltube swaged on the optical fiber, said metal tube crimping the opticalfiber with the buffer coating of the optical fiber yielding to theswaged metal tube so as to prevent the optical fiber from slippingtherein.
 2. An optical fiber as defined in claim 13, wherein said metaltube is stainless steel.
 3. An optical fiber as defined in claim 13,wherein said metal tube has an internal diameter of about 0.015 inches.4. An optical fiber as defined in claim 13, wherein said metal tube hasan outer diameter of about 0.030 inch.
 5. An optical fiber as defined inclaim 13, further comprising alumina packed around the optical fiberinside the metal tube.